Field of the Invention
1. The present invention relates to a contact apparatus for a network according to digital interface standards to be applied to a home electric apparatus.
2. Discussion of Related Art
Accompanying the recent development of multimedia computers and other similar electronic devices, the network system has been widely expanded. Various kinds of network environments have been developed for personal computers. At home, people can enjoy multimedia services which include audio and video devices (hereinafter called AV devices). For example, an interactive television receiver has been developed as a television receiver having the next generation of multimedia services, which includes a CPU and its control software.
However, while interactive televisions and the like have developed, the present network has failed to properly accommodate the next generation of multimedia services. For example, the interactive television is such an apparatus that cannot be connected to the present network bus. In order to accommodate the new multimedia devices, unified standards for digital interface systems for data transmission and reception between a computer and a digital imaging apparatus have been under discussion. As a low cost peripheral interface suitable for multimedia services, the IEEE (The Institute of Electrical and Electronics Engineers) 1394 interface is presently regarded as very promising.
The transmission of multimedia through multiple channels is possible using the IEEE 1394 interface. The IEEE 1394 interface includes an isochronous transmission function which assures the transmission of video and audio data, or the like, within a specified time. Therefore, the IEEE 1394 interface is a digital interface suitable for image transmission.
At present, the standards for the IEEE 1394 interface are being revised and extended under the leadership of the 1394 Trade Association (1394 TA). The IEEE 1394 interface was originally based on computer technology. However, because synchronous communication is possible through such an interface, AV device manufacturers now participate in the preparation of the above-mentioned standards. The proposal of the Digital VCR Conference is now under discussion at the 1394 TA. The EIAR 4.1, or the like, for the purpose of discussing DVB decoders (corresponding to European digital broadcasting) and USA DTV (Digital TV) decoders, has decided to adopt the IEEE 1394 as the interface standard for multimedia services. See, for example, "Comparison of Three New Interfaces Based on Design Concept for Post SCSI" published in Nikkei Electronics 1994. 7.4 (No. 612) pp. 152-163 for the IEEE 1394 standard.
FIGS. 1 and 2 illustrate the interface system of the IEEE 1394 which can perform synchronous transmission and transmission through a plurality of channels. As described on page 162 and thereafter of the Nkkei Electronics article discussed above, multichannel transmission is possible by using the IEEE 1394. FIG. 2 shows an example of data transmission through two channels, namely, channels 1 and 2 (CH1 and CH2) by using a bus corresponding to the IEEE 1394 standard (hereinafter called IEEE 1394 bus). The IEEE 1394 bus can adopt daisy chain topology and tree topology. FIG. 1 illustrates a plurality of devices A through D connected in the form of a daisy chain by means of the IEEE 1394 cable 40 as the IEEE 1394 bus. The devices A through D may be digital VCRs, or other AV devices.
FIG. 2 shows an example of the data transmission from device A to device C, and at the same time from device B to device D. The output from the device A is dubbed at the device C, and at the same time the output from the device B is dubbed at the device D. In the IEEE 1394, data is transmitted at isochronous cycles of 125 .mu.s.
Line (a) of FIG. 2 illustrates a video stream of the dubbing output from the device A. Such dubbing output is transmitted in each isochronous cycle. Line (c) of FIG. 2 shows a video stream of the dubbing output from the device B. This dubbing output is also transmitted in each isochronous cycle. A plurality of channels are assigned to each isochronous packet. The channel number is inserted in the packet from the devices A and B, which specifies a channel through which the dubbing output is transmitted. Line (e) of FIG. 2 shows the transmission of the output packet from the device A through channel 1 (CH 1) and the transmission of the output packet from the device B through channel 2 (CH2).
The devices A and B output commands shown in lines (b) and (d) of FIG. 2 respectively through the IEEE 1394 cable. As shown in line (e) of FIG. 2, these video streams and commands are multiplexed in each isochronous cycle and transmitted through the IEEE 1394 cable 40. As shown in line (e) of FIG. 2, asynchronous data such as commands, or the like, is multiplexed at each gap between synchronous data (video data) and then transmitted.
The devices C and D determine the transmitted data to be received according to the channel number included in the packet transmitted through the IEEE 1394 cable 40 for its reception. That is, the device C receives the data transmitted through CH 1 and the device D receives the data transmitted through CH2.
As described above, AV devices can be connected with each other through the network bus conforming to the IEEE 1394 standard. It is necessary for an AV device not corresponding to the IEEE 1394 to connect its output devices with its respective input devices through cables. For example, to connect one output device with N input devices, the output device requires at least N cables for the video signal only. With the adoption of the IEEE 1394 digital interface, it is possible to connect the devices in a daisy chain form by installing 1 to 3 IEEE 1394 terminals on each device. As shown in FIG. 1, each device can be connected to up to two other devices through cables 40. Up to 63 devices can be connected by this daisy chain connection.
As described above, with the adoption of the IEEE 1394 digital interface, the complexity of the cable connection can be avoided. However, each device requires an AC power cable in addition to the IEEE 1394 cables, namely, three cables in total.
Generally speaking, since the AC wall outlets are installed at one or several places near the AV devices collectively, it is relatively easy to connect an AC cable with the AC outlet on the wall. However, the IEEE 1394 cable is usually installed on the rear of the devices, so in some cases they must be moved to connect them with each other. As described above, even devices conforming to the IEEE 1394 standards require many cables and their connection is relatively troublesome.
Such cable connection may be simplified by using a cable 40 including the AC cable and the IEEE 1394 cables therein. However, if such a cable is used, only a 1:1 connection is possible.
Incidentally, in IEEE 1394, network settings are reset when the power is turned on. By this resetting process, the connections between devices are confirmed, the master-slave relationship between the devices is set and each device ID is set again.
For the transmission of asynchronous data, such as computer data, resetting the network is not troublesome. However, it becomes a problem for the transmission of image data which must be isochronously transmitted. Since isochronous transmission is not assured by network resetting, the display of image data on a monitor may stop. In the case of VTR dubbing, it is necessary to control VTR recording intermittently. The recording by dubbing is actually impossible. Thus, if power supply to even one device in a network is turned off, the network configuration changes and reconfiguration of the network takes place.